In the editor define a function named map with two generic type parameters, T and U. The function takes two arguments -

Couple of things to note here:

First, when defining the new array of U, you have to declare the new array instead of creating an array and assigning an empty array to it. Arrays are an Objective-C structure and can't be initialized with generic types. It requires a different method to declare it which I've put below.

var newArray = [U]()

Next, you don't have to specify the values the for loop is going to iterate over. You can just say "for (name) in (array)" and the for loop will automatically count the values in the array and stop execution once it reaches the last value.

From the looks of it, you're performing a transformation to your array without using the transformation parameter in the function declaration. All you have to do is pass in the transformation parameter into the append method you're calling on the new array in the loop and then pass in the value from the old array to the transformation function.

Once this is all done, return your new array and everything should pass. I've put my solution below for reference.

func map<T, U>(array: [T], transformation: (T) -> U) -> [U] {
   var newArray = [U]()

    for values in array {
        newArray.append(transformation(values))
    }
    return newArray
}